The invention relates to a zinc/air cell having an anode comprising zinc and an air cathode. The invention relates to adding a protective metal layer onto the peripheral edge and also optionally onto the outside surface of a multiclad anode casing for zinc/air cell.
Zinc/air cells are typically in the form of a miniature button cells which have particular utility as batteries for electronic hearing aids including programmable type hearing aids. Such miniature cells typically have a disk-like cylindrical shape of diameter between about 4 and 12 mm and a height between about 2 and 6 mm. Zinc air cells can also be produced in somewhat larger sizes having a cylindrical casing of size comparable to conventional AAAA, AAA, AA, C and D size Zn/MnO2 alkaline cells as well as rectangular/prismatic cells.
The miniature zinc/air button cell typically comprises an anode casing (anode cup), and a cathode casing (cathode cup). The anode casing and cathode casing each typically have a cup shaped body with integral closed end and opposing open end. After the necessary materials are inserted into the anode and cathode casings, the open end of the anode casing is typically inserted into the open end of the cathode casing with electrical insulating material therebetween and the cell sealed by crimping. The anode casing can be filled with a mixture comprising particulate zinc. Typically, the zinc mixture contains mercury and a gelling agent and becomes gelled when electrolyte is added to the mixture. The electrolyte is usually an aqueous solution of potassium hydroxide, however, other aqueous alkaline electrolytes can be used. The cathode casing contains an air diffuser (air filter) which lines the inside surface of the cathode casing""s closed end. The air diffuser can be selected from a variety of air permeable materials including paper and porous polymeric material. The air diffuser is placed adjacent air holes in the surface of the closed end of the cathode casing. Catalytic material typically comprising a mixture of manganese dioxide, carbon and hydrophobic binder can be inserted into the cathode casing over the air diffuser on the side of the air diffuser not contacting the air holes. An ion permeable separator is typically applied over the catalytic material so that it faces the open end of the cathode casing against the anode material in a crimped cell.
The cathode casing can typically be of nickel plated stainless steel, for example, with the nickel plate forming the cathode casing""s outside surface and stainless steel forming the casing""s inside surface. The anode casing can also be of nickel plated stainless steel, typically with the nickel plate forming the casing""s outside surface. The anode casing can be of a triclad material composed of stainless steel having an outer layer of nickel and an inner layer of copper. In such embodiment the nickel layer typically forms the anode casing""s outside surface and the copper layer forms the anode casing""s inside surface. The copper inside layer is desirable in that it provides a highly conductive pathway between the zinc particles and the cell""s negative terminal at the closed end of the anode casing. The triclad (or other multiple clad) anode casing can be formed by plating one metal onto the other or more preferably by heat/pressure forming (cladding) one metal onto the other preferably before the casing has been shaped. An insulator material typically in the form of a ring or disk of a durable, polymeric material can be inserted over the outside surface of the anode casing. The insulator ring is typically of high density polyethylene, polypropylene or nylon which resists flow (cold flow) when squeezed.
After the anode casing is filled with the zinc mixture and electrolyte and after the air diffuser, catalyst, and ion permeable separator is placed into the cathode casing, the open end of the anode casing can be inserted into the open end of the cathode casing. The peripheral edge of the cathode casing can then be crimped over the peripheral edge of the anode casing to form a tightly sealed cell. The insulator ring around the anode casing prevents electrical contact between the anode and cathode cups. A removable tab is placed over the air holes on the surface of the cathode casing. Before use, the tab is removed to expose the air holes allowing air to ingress and activate the cell. A portion of the closed end of the anode casing can function as the cell""s negative terminal and a portion of the closed end of the cathode casing can function as the cell""s positive terminal.
Typically, mercury is added in amount of at least one percent by weight, for example, about 3 percent by weight of the zinc in the anode mix. The mercury is added to the anode mix to improve inter-particle contact between zinc particles in the anode mixture. This in turn improves electrical conductivity within the anode and thus results in increased cell performance, for example, higher actual specific capacity of the zinc (Amp-hour/g). Also addition of mercury tends to reduce the hydrogen gassing which can occur in the zinc/air cell during discharge and when the cell is placed in storage before or after discharge. The gassing, if excessive, increases the chance of electrolyte leakage, which can damage or destroy the hearing aid or other electronic component being powered. It is desirable to reduce the amount of added mercury or eliminate adding mercury to the anode, since many regions around the world now restrict the use of mercury in electrochemical cells because of environmental concerns.
There can occasionally be creep of some electrolyte in the seal area provided between the anode and cathode casing thereby resulting in some electrolyte seepage from the cell. Such electrolyte seepage can occur regardless of whether mercury has been added to the anode. However, zinc/air cells that contain reduced amount of mercury, e.g. less than 3 percent by weight mercury based on the zinc or zero added mercury are generally more prone to gassing and such electrolyte creep. Seals have been provided wherein the insulating disk separating the anode and cathode casing has been coated on its inside surface (the insulator surface facing the anode casing) with an asphalt or polymeric sealant paste or liquid. However, this does not completely solve the problem of electrolyte creep between the anode and cathode casing in all circumstances. The electrolyte seepage can be promoted by surface imperfections on anode casing outer surfaces as well as the mating insulator surfaces. Misuse of the cell, that is, discharging the cell at higher current drain than intended can also promote excessive seepage.
The anode casing is typically formed of a triclad metal comprising a stainless steel body plated on the outside with a layer of nickel and on the inside with a layer of copper. The peripheral edge of the anode casing typically is clipped resulting in surface exposure of the three metals in very close proximity (within the thickness of the anode casing). It is believed that the exposure of the electrolyte to the different metals at the anode casing peripheral edge results in an electrochemical potential gradient causing surface reactions which in turn promotes electrolyte creep.
U.S. Pat. No. 3,897,265 discloses a representative zinc/air button cell construction with an anode casing inserted into the cathode casing. There is disclosed an insulator between the anode and cathode casings. The anode comprises zinc amalgamated with mercury. The cell includes an assembly comprising an air diffuser, cathode catalyst, and separator at the closed end of the cathode casing facing air holes in the surface of the cathode casing.
U.S. Pat. Nos. 5,279,905 and 5,306,580 disclose a miniature zinc/air cell wherein little or no mercury has been added to the anode mix. Instead, the inner layer of the anode casing has been coated with a layer of indium. The disclosed anode casing can be a triclad material composed of stainless steel plated on the outside surface with nickel and on the inside surface with copper. The copper layer is at least 1 microinch (25.4xc3x9710xe2x88x926 mm). The reference discloses coating the copper layer on the anode casing""s inside surface with a layer of indium. The indium layer is disclosed as being between about 1 microinch and 5 microinches (25.4xc3x9710xe2x88x926 mm and 127xc3x9710xe2x88x926 mm).
The invention is applicable to a zinc/air cell, particularly a zinc/air button cell having an anode casing which in inserted into the open end of a cathode casing. The invention is applicable to anode casings which are multiclad, that is, formed of two or more layers of metals. The zinc/air button cells have particular application as batteries for hearing aids. The anode casing typically has a cylindrical shape having an integrally formed closed end and opposing open end. The anode casing thus forms a cup shape, which is filled with anode material comprising a mixture of zinc and alkaline electrolyte, preferably an aqueous solution of potassium hydroxide. The anode casing is inserted into the open end of a larger cup shaped cathode casing with insulating material therebetween. The anode casing is desirably formed of a tri-clad material comprising a stainless steel body which is plated on its outside surface with nickel and on the inside surface with copper. Thus, with such tri-clad the copper layer of the anode casing faces the anode material comprising zinc. The closed end of the cathode has air holes which allows air to pass through a catalytic layer comprising MnO2, preferably a mixture of MnO2 and carbon, within the cathode casing. Thus the zinc/air cell functions through electrochemical reaction wherein zinc oxidizes at the anode releasing electrons, while incoming oxygen at the cathode is reduced by absorbing electrons.
When the anode casing is formed of a multiclad metal, for example a triclad of nickel/stainless steel/copper, the peripheral edge of the anode casing has exposed along its surface each of the individual metals nickel, stainless steel, and copper in close proximity to each other, that is within the thickness of the anode casing, e.g. between about 0.001 inches and 0.015 inches (0.0254 mm and 0.38 mm). The close proximity of the exposed metal layers along the anode casing peripheral edge produces electrochemical potential gradients when in contact with electrolyte. Such potential gradient can cause secondary reactions which in turn can promote electrolyte creep along the outside surface of the anode casing, that is, between the anode and seal. The difference in hydrogen overpotential between these different metals is believed to be a significant contributor. In such circumstances electrolyte creep can occur despite the presence of tightly placed insulating material forming a seal between the anode and cathode casing.
A principal aspect of the invention is directed to reducing, preferably eliminating, the potential gradients at the peripheral edge of the anode casing caused by the use of a multi-clad anode casing. In accordance with the invention the exposed peripheral edge of the anode casing is plated with a protective metal , preferably selected from tin, indium, silver, copper, brass, bronze, phosphor bronze, silicon bronze, tin/lead alloy (alloy of the combination tin and lead) and gold. Desirably the protective metal can be tin, indium, silver or copper, more preferably tin or copper after the anode casing has been formed (post plating). The single metal is preferably pure elemental metal, but can also be an alloy containing tin, indium, silver, copper or gold, for example, brass (an alloy of copper and zinc) or bronze (an alloy of copper and tin) or an alloy of the combination tin and lead (Sn/Pb). The tin, indium, silver or copper, for example, can also be the principal component and the alloy metal can comprise preferably less than about 500 parts per million parts by weight of the total alloy. Metals which are essentially composed of tin, indium, silver, or copper comprising such small amount of alloy additive do not noticeably diminish the principal function of the protective metal, which is to markedly reduce, preferably eliminate, the potential gradient across the surface of the anode casing peripheral edge. Similarly, the protective metal can be composed of brass (an alloy of copper and zinc) or bronze (an alloy of copper and tin), phosphor bronze or silcon bronze or alloy of tin and lead (Sn/Pb). Such metals or metal alloys when applied homogenously over the peripheral edge of the anode casing essentially eliminate the potential gradient across said peripheral edge.
Thus, in a principal aspect of the invention, if the anode casing is formed of a multiclad, for example, the triclad nickel/stainless steel/copper, then the peripheral edge of the anode casing exposing each of these materials is post plated with a metal preferably tin, indium, silver copper, or gold more preferably tin or copper, brass, bronze, phosphor bronze or silicon bronze or tin/lead alloy. The protective metal should be applied so that its composition is homogeneous across the surface of the exposed peripheral edge of the anode casing and it is essentially pin-hole free to the extent practical. Preferably, the protective metal is applied to a thickness of between about 0.0001 and 0.015 mm. This in effect minimizes/eliminates the potential gradients caused by exposure of the individual (multiclad) metals along the peripheral edge of the anode casing and minimizes local gassing and hydroxyl ion [OH]xe2x88x92 formation which are believed to enhance creep of electrolyte. In turn the problem of electrolyte seepage between the anode and cathode casing is thereby alleviated or eliminated altogether. The reduction of electrolyte leakage is accomplished by the present invention irrespective of whether the cell contains added mercury (e.g. containing between 1 and 3 percent by weight mercury) or is essentially mercury free (e.g. less than 100 parts mercury per million parts zinc). The effect is more pronounced and the efficacy of the current invention more easily observable in an essentially mercury-free battery.
In another aspect of the invention in addition to post plating the protective metal of the invention onto the exposed peripheral edge of the anode casing, such plating of the protective metal can also extend from said peripheral edge to also cover at least a portion and preferably all of the outside surface of anode casing abutting the insulating material between anode and cathode casing. Additionally, the protective metal of the invention can be applied to cover the entire outside surface of the anode casing including the clipped peripheral edge.
In another aspect it will be appreciated that the exposed peripheral edge of the anode casing and desirably at least a portion of the outside surface of the anode casing extending from said peripheral edge, can be plated with two or more layers of metals stacked one on top of the other. Such multiple layers can each be formed of the same or different metals, preferably selected from tin, indium, silver or copper, (or above discussed alloys thereof) or can be zinc as one of the intermediate layers or an alloy of tin and lead (Sn/Pb alloy) or can be alloys such as brass, bronze, phosphor bronze or silicon bronze. However, if multiple layers are applied it is desired that each layer be applied in a homogenous composition pin-hole free and preferably in an even thickness. This assures that there is no noticeable electrochemical potential gradient along the outer surface (last applied layer) of the protective metal. For example, the initial plating layer over the exposed triclad material can be silver. A layer of tin can then be plated over the silver. In such case each layer desirably has a thickness between about 0.0001 and 0.015 mm, for example, between about 0.0001 and 0.010 mm. The resulting effect is that the exposed surface of the anode casing peripheral edge is a single metal of homogenous composition, e.g. tin thereby reducing or eliminating the potential gradient along the surface said peripheral edge.
Another aspect is directed to post plating the exposed peripheral edge of the anode casing in accordance with the invention irrespective of whether the multiclad anode casing is formed of dual clad, triclad or quater (four) clad material. The invention is thus not intended to be limited to anode casing formed of a triclad material and is not intended to be limited to any specific metals which form the multiclad anode casing.